1. Field of the Invention
This invention relates in general to flow rate measuring devices using ultrasound wherein the phase of ultrasound signals which travel through the fluid in the upstream and downstream direction are different.
2. Description of the Prior Art
Prior art methods and apparatus for measuring flow rates using ultrasound based upon phase differences of the ultrasound signals have the disadvantages that
1) three ultrasound transducers must be utilized,
2) the upstream and downstream measuring paths are not identical, and
3) the zero phase is usually not specified in the integrated circuits utilized and the circuits must therefore be individually balanced. It is thus not assured that the balancing will be maintained when temperature changes and aging effects occur.
German published application No. 27 24 661 avoids the disadvantages 1 and 2 listed above by providing that clocked sound signals which are referred to as "sound bursts" of identical sound frequencies are simultaneously emitted by both transducers. The two "sound bursts" travel the measuring paths in opposite directions and impinge upon the other sound transducer almost simultaneously and are converted back into electrical signals. The chronological lengths t.sub.B of the sound burst is selected such that the sound transducers are no longer transmitting at the arrival time of the received signals: EQU t.sub.B .ltoreq.L/c (1)
where L represents the length of the measuring path and c represents the speed of sound.
The phase difference .DELTA..phi. of the two signals which travel in different directions is a measure of the flow rate v: ##EQU1## where f is the frequency of the transmitted signals.